The present invention relates to a circuit arrangement with an adjustable amplitude-frequency response between an input, which is arranged to receive an input signal, and an output for taking off an output signal.
DE-PS 25 37 140 describes a method of correcting the amplitude response of an FM radio channel of a radio-relay system without the phase response and, directly associated therewith, the envelope delay, being adversely affected, shunted T networks being used as all-pass filters. In order to provide correction for the attenuation of the delay equalizer in the transmission band, this method, in contradistinction to the dimensioning customary for an idealised all-pass filter with loss-free elements where the attenuation is constant as a function of the frequency, provides a parabolic roll-off relative to this constant attenuation towards smaller values in a manner such that the degree of the parabola corresponds to the degree of the distortion. However, such an arrangement only provides an approximate independence of the phase response of the amplitude response merely for a fixed amplitude and a fixed amplitude response, i.e. for so-called all-pass filters.
EP-PS 11 446 describes a circuit arrangement with an adjustable frequency response, comprising a first filter circuit, a first amplifier to amplify the output signal of the filter circuit, a second amplifier to amplify a signal which forms the difference between the input signal and the output signal of the filter circuit, and an adder to add the output signals of the first and the second amplifier. In this known circuit arrangement the filter circuit further comprises a capacitor and a variable-gain amplifier, at least the first or the second amplifier being a variable-gain amplifier, and also includes means for generating a gain control signal, which means are connected to the first and/or the second amplifier forming the variable-gain amplifier(s) and the variable-gain amplifier of the filter circuit, to generate a control signal for controlling the gain factor of the first or the second amplifier and at the same time adjusting the cut-off frequency of the filter circuit. However, such a circuit arrangement does not provide a phase response independent of the frequency response.
JP-A-62-285 508 discloses a circuit arrangement by means of which the gain at a given frequency can be varied without a variation of the phase response and the delay characteristic. This known circuit arrangement comprises a first, a second and a third differential amplifier. An input signal is applied from a signal input to the non-inverting inputs of the first and the second differential amplifier. A signal from the non-inverting output of the second differential amplifier is applied both to a grounded capacitor and to the non-inverting input of the third differential amplifier. The inverting output of the first differential amplifier and the non-inverting output of the third differential amplifier are connected to a signal output, which is also connected to all of the inverting inputs of the three differential amplifiers and, in addition, to the signal input via a second capacitor. The slope of the first differential amplifier can be controlled by means of a control circuit.
However, this circuit arrangement is invariably based on a given transfer characteristic. There are no suggestions to adapt this known circuit arrangement so as to obtain different transfer characteristics, for example, a high-pass or a low-pass characteristic. Moreover, the multiple feedback from the output terminal to the various inputs of the differential amplifiers results in an unpredictable oscillation tendency of the arrangement.
Moreover, GB-A 2,209,642 discloses different circuit arrangements by means of which a signal is to be modified in an adjustment mode or should not be influenced in another mode, in which last-mentioned mode neither the amplitude nor the phase of the input signal should be changed. In these circuit arrangements the gain factors of the various operational amplifiers used therein are varied relative to one another in accordance with a given relationship in order to modify the gain or phase characteristics of the individual circuits independently of one another or, in the other mode, they are equalised or set to unity.
The circuit arrangements mentioned here resemble the arrangement in accordance with JP-A-62-285 508 as regards their construction and properties and consequently have the same drawbacks.